Carbon containing tips with cylindrically symmetrical carbon containing expanded bases

ABSTRACT

Systems and methods are described for carbon containing tips with cylindrically symmetrical carbon containing expanded bases. A method includes producing an expanded based carbon containing tip including: fabricating a carbon containing expanded base on a substrate; and then fabricating a carbon containing fiber on the expanded base. An apparatus includes a carbon containing expanded base coupled to a substrate; and a carbon containing extension coupled to said carbon containing expanded base. The carbon containing expanded base is substantially cylindrically symmetrical and said carbon containing extension is substantially cylindrically symmetrical.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims a benefit of priorityunder 35 U.S.C. 120 from U.S. Ser. No. 09/795.660, filed Feb. 27, 2001now U.S. Pat. No. 6,649,431, the entire contents of which are herebyexpressly incorporated by reference for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with United States Government support undercontract to UT-Battelle, LLC. The Government has certain rights in thisinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of carbon tips. Moreparticularly, a preferred implementation of the invention relates tocarbon tips with expanded bases. The invention thus relates to carbontips of the type that can be termed expanded base.

2. Discussion of the Related Art

There are many technological/scientific tools and devices that utilizesharp tips with high aspect ratio. They include scanning probemicroscopy, biological probes, field emission (FE) devices, etc. Tiprequirements for the above applications include extreme sharpness and ahigh aspect ratio, combined with mechanical stability as well aschemical inertness and resistance to sputtering. The preferred tip shapeis a cylinder because it enables higher resolution when measuring highaspect ratio features and also provides a substantially higher fieldenhancement at the apex of the tip as compared to other geometries,thereby enabling field emission of electrons at low applied fields.

Vertically-aligned carbon nanotubes (VACNTs) and nanofibers (VACNFs) arematerials that possess a number of unique properties that make them wellsuited for the use as tips. First, VACNTs and VACNFs have intrinsicallysmall diameters (˜1 nm for single-wall CNTs). Second, VACNTs/VACNFs havevery high aspect ratios and cylindrical shape. Third, VACNTs/VACNFsexhibit high mechanical strength and flexibility. Fourth, some VACNTsand all VACNFs are electrically conducting which permits their use whencurrent measurements and electron field emission are involved (scanningtunneling microscopy, FE (field effect) devices, biological probes).Fifth, VACNTs/VACNFs are chemically inert and highly sputter resistant.Finally, VACNFs can be grown at predetermined locations (deterministicgrowth), which enables their incorporation into actual devices.

The growth process of forests of randomly placed vertically alignedcarbon nanofibers (VACNFs) was first pioneered by Ren et al. (1). Lateron, our group (2) and Ren et al. (3) independently developed a methodfor deterministic growth of individual VACNFs.

However, there is a significant obstacle associated with the use ofhigh-aspect-ratio cylindrical tips. As the aspect ratio increases, thevery ends of these tips exhibit significant thermal and mechanicalvibrations. Therefore, what is needed is a solution that provides ahigh-aspect-ratio cylindrical tip that is thermal and mechanicalvibration resistant. What is also needed are carbon tips having improvedquality and reduced cost.

SUMMARY OF THE INVENTION

There is a need for the following embodiments. Of course, the inventionis not limited to these embodiments.

One embodiment of the invention is based on a method, comprisingproducing an expanded base carbon containing tip including: fabricatinga carbon containing expanded base on a substrate; and then fabricating acarbon containing extension on the expanded base. Another embodiment ofthe invention is based on an apparatus, comprising: a carbon containingexpanded base coupled to a substrate; and a carbon containing extensioncoupled to said carbon containing expanded base.

These, and other, embodiments of the invention will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingvarious embodiments of the invention and numerous specific detailsthereof, is given by way of illustration and not of limitation. Manysubstitutions, modifications, additions and/or rearrangements may bemade within the scope of the invention without departing from the spiritthereof, and the invention includes all such substitutions,modifications, additions and/or rearrangements.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. A clearerconception of the invention, and of the components and operation ofsystems provided with the invention, will become more readily apparentby referring to the exemplary, and therefore nonlimiting, embodimentsillustrated in the drawings, wherein like reference numerals designatethe same elements. The invention may be better understood by referenceto one or more of these drawings in combination with the descriptionpresented herein. It should be noted that the features illustrated inthe drawings are not necessarily drawn to scale.

FIGS. 1A–1F illustrate schematic views of a process, representing anembodiment of the invention.

FIGS. 2A–2D illustrate scanning electron micrographs of single (b and d)and multiple (a and c) vertically aligned carbon nanofibers,representing embodiments of the invention.

FIGS. 3A–3B illustrate schematic views of a process, representing anembodiment of the invention.

FIGS. 4A–4C illustrate scanning electron micrographs of carbonnanocones, representing embodiments of the invention.

FIGS. 5A–5B illustrate scanning electron micrographs of cylinder-on-conecarbon tips, representing embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention and the various features and advantageous details thereofare explained more fully with reference to the nonlimiting embodimentsthat are illustrated in the accompanying drawings and detailed in thefollowing description. Descriptions of well known components andprocessing techniques are omitted so as not to unnecessarily obscure theInvention in detail. It should be understood, however, that the detaileddescription and the specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only andnot by way of limitation. Various substitutions, modifications,additions and/or rearrangements within the spirit and/or scope of theunderlying inventive concept will become apparent to those skilled inthe art from this detailed description.

Within this application several publications are referenced bysuperscripts composed of Arabic numerals within parentheses. Fullcitations for these, and other, publications may be found at the end ofthe specification immediately preceding the claims after the sectionheading References. The disclosures of all these publications in theirentireties are hereby expressly incorporated by reference herein for thepurpose of indicating the background of the invention and illustratingthe state of the art.

The below-referenced U.S. Patent, and U.S. Patent Application discloseembodiments that were satisfactory for the purposes for which they areintended. The entire contents of U.S. Pat. No. 6,078,392 are herebyexpressly incorporated by reference herein for all purposes. The entirecontents of U.S. patent application Ser. No. 09/368,919, filed Aug. 5,1999 are hereby expressly incorporated by reference herein for allpurposes.

A conically shaped tip has fewer thermal and mechanical vibrationproblems compared to a cylindrical tip due to the much higher mechanicaland thermal stability provided by the wider base. However, compared to acylindrical tip, a conically shaped tip has substantially lowerresolution for measuring high aspect ratio features and requires higherelectric fields for electron emission. Therefore, the truly ideal tipshape is a cylinder sitting on top of a cone. This cylinder-on-conestructure combines mechanical stability of the cone and the advantage ofthe cylinder for high resolution measurements and field emission at lowapplied fields, thus yielding a perfect tip shape.

The invention can include one or more nanoscale carbon tips withcylinder-on-cone shapes. The phrase cylinder-on-cone is defined as acomposite shape composed of a substantially cylindrical symmetric rodthat extends substantially coaxially away from near the apex of asubstantially cylindrically symmetric cone. These composite shapes canbe solid (cone and rods) and/or hollow (funnels and tubes).

The invention can include a method for fabrication of nanoscalecylinder-on-cone carbon shapes that can be used as tips in a variety ofapplications. Perpendicularly oriented nanoscale cylinder-on-cone carbontips (NCCCTs) can be grown directly at predetermined locations on asubstrate. This method of production allows for fabrication ofsubstantially ideally shaped tips with precise control of position. Thismethod of production is well suited for large-scale commercialproduction. This method can utilize a combination of electron-beamlithography (EBL) and plasma-enhanced chemical vapor deposition(PE-CVD). Both of these techniques are well developed, do not requirecomplicated manipulations, and are well suited for large-scalecommercial production. The fabrication process allows growth of NCCCTsat pre-determined locations on a substrate and yields mechanicallystrong connection between the base of NCCCTs and the substrate. It alsoenables precise and accurate control of the crucial NCCCT parameterssuch as the length of the cone and the cylinder, the cone angle, and thediameter of the NCCCT apex. Consequently, NCCCTs can be tailored toperfectly suit specific applications.

The method can include two main steps: formation of a precisely placedsingle metal catalyst dot on a substrate by utilizing EBL in conjunctionwith electron-gun metal evaporation; and catalytic growth of a NCCCT onthe substrate using PE-CVD.

Growth of CNTs and CNFs requires the presence of a catalytic precursor.Ni can be used as a catalyst. However, other metals such as Fe, Co, etc.can also be utilized as the catalyst with these procedures, the resultsbeing similar.

In order to grow a single carbon nanocone (CNC) or CNF, formation of asingle catalyst nanoparticle (or cluster) may be required. This catalystnanoparticle is formed by forming a metal catalyst dot with width(diameter) D, and film thickness, T. FIGS. 1A–1F show fabrication ofvertically aligned carbon nanofibers (VACNFs) utilizing plasma enhancedchemical vapor deposition (FIGS. 1E–1F) preceded by electron-beamlithography and metal evaporation (FIGS. 1A–1D). A catalyst dot 150 isfabricated on a substrate using electron beam (e-beam) lithography andelectron gun (e-gun) metal evaporation as shown in FIG. 1A–1D. Asubstrate 110 is first coated with an e-beam resist 120 (e.g. PMMA); theresist 120 is then e-beam exposed and developed (FIG. 1A and 1B), toproduce a small opening 130 in the resist 120 with width (diameter) D. Abuffer layer 140 (Ti in this case) is deposited next to prevent theformation of catalyst silicide and to impede catalyst diffusion atelevated temperatures. Next, a catalyst layer 150 (Ni, in this case) isdeposited (FIG. 1C). Finally, a single, isolated catalyst dot isobtained by lifting off the metal-coated resist in acetone (FIG. 1D).Multiple dots, or pattern arrays of dots, also can be produced by thisprocess.

FIGS. 1E–1F show a VACNF is prepared by PECVD in a vacuum chamber 160.The vacuum chamber 160 can include an anode 162 and a cathode 164. Thecathode 164 can also function as a heater. One of the advantages of thismethod is that high vacuum is not required, i.e. the chamber 160 can beevacuated using only a mechanical pump (FIG. 1E and 1F). Upon ammoniaplasma pre-etching (alternatively, hydrogen and other gases can be used)and annealing the Ni/Ti assembly on Si at the elevated temperaturesrequired to grow VACNFs (˜700° C. in this case), the Ti layer 140continues to adhere to the Si substrate 110, whereas the initiallycontinuous Ni layer 150 breaks into one or more little nanoparticledroplets (FIG. 1E).

FIGS. 2A–2D show scanning electron microscopy images of single andmultiple vertically aligned carbon nanofibers formed from a single ormultiple catalyst dot. Upon heating and ammonia plasma pre-etching thecatalyst layer breaks into nanodroplets. Each nanodroplet provides forcatalytic growth of an individual nanofiber. This droplet is thenecessary precursor for the catalytic growth of a single VACNF at thispredetermined location. Within an initially large dot, multiple dropletsare formed (FIG. 2A). See FIG. 2B also. However, below a critical dotsize only a single nanoscale Ni droplet forms (FIG. 2B) and consequentlyonly a single nanofiber is grown. See FIG. 2D also. The critical dotsize, and the size of its resulting Ni droplet, will depend upon thechoice of the buffer layer between the catalyst and the substrate, thetype and thickness of the catalyst used, and the annealing/growthtemperature. For example, for growth of a single VACNF at 700° C. usingan initially 15 nm thick Ni catalyst on a Ti buffer layer on Si, thecritical dot size (diameter) is ˜350 nm. The diameter of the Ninanoparticle droplet formed is about a factor of 3 smaller, ˜100 nm inthis example. Smaller catalyst nanoparticles can be obtained byinitially forming a smaller catalyst dot. For instance, 100 nm dots witha 10 nm thick Ni layer produce Ni droplet of 30–40 nm in diameter, and50 nm dots yield 20–30 nm droplets.

For VACNF growth, a mixture of a carbonaceous gas and an etchant (e.g.acetylene and ammonia) can be used as the gas source. The etchant isneeded to etch away graphitic carbon film that continuously forms duringthe growth from the plasma discharge. If not removed, the role of thefilm will be passivating the catalyst and thereby preventing theformation of VACNFs. The invention can include heating the substratesdirectly by placing them on a heater plate (e.g., the cathode of theplasma discharge). This technique has the advantages that (i) it caneasily be scaled up for large-area deposition and (ii) the substratetemperature is known and easily controlled. For example, an array ofdots could be patterned as described here and then placed on the heaterplate to simultaneously grow all of the (highly uniform) VACNFs.

Just prior to the VACNF growth process, ammonia can be introduced intothe chamber and a plasma created (FIG. 1F). The invention can utilize adc (direct current) glow discharge plasma. However, radio-frequency (rf)or microwave plasmas also can be employed. After the plasma is started,acetylene can be introduced and the VACNF growth can begin. Each Ni(nickel) droplet initiates the formation of an individual VACNF (FIGS.2C and 2D). The Ni droplet can reside on top of the VACNF and providesfor its continued catalytic growth upwards (4). The VACNFs are orientedalong plasma field lines and normally grow perpendicular to thesubstrate.

The invention can include adjusting the growth parameters, such as theratio of acetylene to ammonia. In this way, a CNC rather than a CNF canbe formed. Herein the word “nano” is referred to the tip diameter of theCNCs; the CNC height and base diameter can be grown to a _(μ)m size. Ifthe acetylene content is increased relative to that of ammonia (inaddition to just diffusing through the Ni particle and precipitating atits bottom, thus providing for the growth in the vertical direction)carbon also begins to precipitate at the walls of the growing, initiallycylindrical VACNF. Precipitation occurs due to the insufficient amountof the etchant (ammonia), which leads to the deposition rate of carbonbeing higher than the etching rate. Thus growth in two dimensions(vertical due to the catalytic growth through the Ni particle andlateral due to the carbon precipitation at the walls) occurs. As aresult, a conical structure forms, as shown in FIG. 3A. The tip diameterof the cone remains constant during the growth process and is determinedonly by the size of the catalyst droplet. In contrast, at a givenacetylene content the base diameter of the CNC increases with growthtime. Furthermore, by changing growth parameters, such as the relativeacetylene content, the cone angle can be changed. Higher acetylenecontent and higher pressure yield higher cone angles and vice versa TheCNC height is proportional to the growth time.

The invention can also include a process in which a carbon cylinder(VACNF) is grown directly on a CNC in situ by changing the growthparameters during the synthesis process. As soon as a CNC of desiredlength and shape (cone angle) is obtained, the relative acetylenecontent in the chamber can be reduced to suppress the carbon growth inthe lateral dimension, thus yielding the formation of a regular VACNFthat has cylindrical shape and resides on top of the CNC. Again, the tipdiameter of the VACNF is determined only by the catalyst droplet sizeand the length is controlled by the growth time.

FIG. 3A shows growth of carbon nanocones in excess of acetylene. FIG. 3Bshows subsequent growth of carbon nanofibers at the ends of thenanocones as the relative acetylene content is reduced during the growthprocess.

A method for controlled synthesis of NCCCTs can include three steps. Thefirst step can be formation of a precisely placed single metal catalystdot (or an array of dots) on a substrate by utilizing EBL in conjunctionwith electron-gun metal evaporation. The second step can be catalyticgrowth of a vertically oriented carbon nanocone on the substrate usingPECVD with excess of a carbonaceous gas (e.g., acetylene in this work).The third step can be changing the growth parameters (e.g., relativeacetylene content) during the growth process to synthesize ananocylinder (VACNF) at the end of the nanocone.

The method enables substantially completely deterministic growth of CCCNtips, as the location of the tip, the length of the cone and thecylinder, the cone angle, and the diameter of the cylindrical part canall be controlled. Also, an important aspect of this process is itsscalability for large-scale synthesis, which enables commercial massproduction.

The invention can include the use of different carbon source gasses(e.g., ethelyne or methaene ). Similarly, the invention can include theuse of different etchant gases (e.g., hydrogen). The key is to have asource of carbon and an etchant.

The transition from expanded base growth to fiber growth can be effectedby changing growth parameters other than the relative acetylene content,e.g. plasma power, discharge voltage, total pressure in the chamber,land the˜growth temperature, can be used to change the shape of the tip(nancone-to-nanofiber transition). The key is to adjust the growthparameters to achieve carbon precipitation at the walls of the growingcarbon nanofiber, thereby creating a cylindrical structure.

The invention can also utilize data processing methods that transformsignals from the structures being grown to control the growth process.For example, the invention can be combined with instrumentation toobtain state variable information to actuate interconnected discretehardware elements. For instance, the invention can include the use ofdata from detecting a laser beam reflected at the tips to control thegrowth of the tips. Similarly, the invention can include the use ofelectron emission detection data to control the growth of the tips.

The term approximately, as used herein, is defined as at least close toa given value (e.g., preferably within 10% of, more preferably within 1%of, and most preferably within 0.1% of). The term substantially, as usedherein, is defined as at least approaching a given state (e.g.,preferably within 10% of, more preferably within 1% of, and mostpreferably within 0.1% of). The term coupled, as used herein, is definedas connected, although not necessarily directly, and not necessarilymechanically. The term deploying, as used herein, is defined asdesigning, building, shipping, installing and/or operating. The termmeans, as used herein, is defined as hardware, firmware and/or softwarefor achieving a result. The term program or phrase computer program, asused herein, is defined as a sequence of instructions designed forexecution on a computer system. A program, or computer program, mayinclude a subroutine, a function, a procedure, an object method, anobject implementation, an executable application, an applet, a servlet,a source code, an object code, a shared library/dynamic load libraryand/or other sequence of instructions designed for execution on acomputer system. The terms including and/or having, as used herein, aredefined as comprising (i.e., open language). The terms a or an, as usedherein, are defined as one or more than one. The term another, as usedherein, is defined as at least a second or more.

The particular manufacturing process used for expanded base tips shouldbe inexpensive and reproducible. Conveniently, the expanded base tips ofthe invention can be carried out by using any vacuum deposition method.It is preferred that the process be chemical vapor deposition. For themanufacturing operation, it is an advantage to employ a plasma enhancedtechnique.

However, the particular manufacturing process used for the expanded basetips is not essential to the invention as long as it provides thedescribed functionality. Normally those who make or use the inventionwill select the manufacturing process based upon tooling and energyrequirements, the expected application requirements of the finalproduct, and the demands of the overall manufacturing process.

The particular material used for the expanded base tips should besubstantially pure. Conveniently, the expanded base tips of theinvention can be made of any source of carbon material. It is preferredthat the material be gaseous. For the manufacturing operation, it is anadvantage to employ an alkene or alkine material.

However, the particular material selected for producing the expandedbase tips is not essential to the invention, as long as it provides thedescribed function. Normally, those who make or use the invention willselect the best commercially available material based upon the economicsof cost and availability, the expected application requirements of thefinal product, and the demands of the overall manufacturing process.

The disclosed embodiments show a plasma discharge vacuum chamber as thestructure for performing the function of fabricating the expanded basetips, but the structure for fabrication of tips can be any otherstructure capable of performing the function of fabrication, including,by way of example a simple chemical vapor deposition chamber or aphysical vapor deposition chamber.

EXAMPLES

Specific embodiments of the invention will now be further described bythe following, nonlimiting examples which will serve to illustrate insome detail various features. The following examples are included tofacilitate an understanding of ways in which the invention may bepracticed. It should be appreciated that the examples which followrepresent embodiments discovered to function well in the practice of theinvention, and thus can be considered to constitute preferred modes forthe practice of the invention. However, it should be appreciated thatmany changes can be made in the exemplary embodiments which aredisclosed while still obtaining like or similar result without departingfrom the spirit and scope of the invention. Accordingly, the examplesshould not be construed as limiting the scope of the invention.

Example 1

An example of a CNC array and CNCs with different cone angles are shownin FIG. 4. FIGS. 4A–4C show an array of carbon nanocones (a) fabricatedusing plasma-enhanced chemical vapor deposition with excess ofcarbonaceous gas (acetylene). Carbon nanocones with (b) large and (c)small cone angles grown with gas flows of 60 sccm C₂H₂/80 sccm NH₃ and55 sccm C₂H₂/80 sccm NH₃, correspondingly.

Example 2

An example of resultant nanoscale cylinder-on-cone carbon tips (NCCCTs)is shown in FIGS. 5A and 5B. FIGS. 5A and 5B show nanoscalecylinder-on-cone carbon tips (NCCCTs) synthesized by first growingcarbon nanocone in excess of acetylene (60 sccm C₂H₂/80 sccm NH₃) andthen growing carbon nanofiber by reducing the relative acetylene content(50 sccm C₂H₂/80 sccm NH₃) during the growth process. Referring to FIG.5A, a carbon containing expanded base 510 includes a precipitatedgraphitic carbon film 520. Growing the carbon containing expanded base510 includes simultaneously applying a carbon source gas and an etchantgas to said carbon containing expanded base 510 and growing said carboncontaining extension includes simultaneously applying the carbon sourcegas and the etchant gas to said carbon containing extension.

Practical Applications of the Invention

Practical applications of the invention that have value within thetechnological arts include atomic force microscopy (AFM), scanningtunneling microscopy (STM), and other scanning probe microscopies; fieldemission devices; biological probes; any other techniques where sharptips are utilized. There are virtually innumerable uses for theinvention, all of which need not be detailed here.

Advantages of the Invention

A carbon tip and/or electron emitter, representing an embodiment of theinvention, can be cost effective and advantageous for at least thefollowing reasons. The invention allows to fabricate nanoscalecylinder-on-cone carbon tips (NCCCTs), not just cylindrical tips such asvertically aligned carbon nanofibers or nanotubes. The crucial tipparameters such as the length of the cone and the cylinder, the coneangle, and the diameter of the cylindrical part can be well controlled.This allows tailoring NCCCTs to meet specific applications. Theinvention improves quality and/or reduces costs compared to previousapproaches.

All the disclosed embodiments of the invention disclosed herein can bemade and used without undue experimentation in light of the disclosure.Although the best mode of carrying out the invention contemplated by theinventor(s) is disclosed, practice of the invention is not limitedthereto. Accordingly, it will be appreciated by those skilled in the artthat the invention may be practiced otherwise than as specificallydescribed herein.

Further, the individual components need not be formed in the disclosedshapes, or combined in the disclosed configurations, but could beprovided in virtually any shapes, and/or combined in virtually anyconfiguration. Further, the individual components need not be fabricatedfrom the disclosed materials, but could be fabricated from virtually anysuitable materials.

Further, variation may be made in the steps or in the sequence of stepscomposing methods described herein. Further, homologous replacements maybe substituted for the substances described herein. Further, agentswhich are chemically related may be substituted for the agents describedherein where the same or similar results would be achieved.

Further, although the carbon tip(s) and/or electron emitter(s) describedherein can be a separate module, it will be manifest that the carbontip(s) and/or electron emitter(s) may be integrated into the system withwhich it is (they are) associated. Furthermore, all the disclosedelements and features of each disclosed embodiment can be combined with,or substituted for, the disclosed elements and features of every otherdisclosed embodiment except where such elements or features are mutuallyexclusive.

It will be manifest that various substitutions, modifications, additionsand/or rearrangements of the features of the invention may be madewithout deviating from the spirit and/or scope of the underlyinginventive concept. It is deemed that the spirit and/or scope of theunderlying inventive concept as defined by the appended claims and theirequivalents cover all such substitutions, modifications, additionsand/or rearrangements.

The appended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase(s) “means for” and/or “stepfor.” Subgeneric embodiments of the invention are delineated by theappended independent claims and their equivalents. Specific embodimentsof the invention are differentiated by the appended dependent claims andtheir equivalents.

REFERENCES

-   1. Ren, Huang, Xu, Wang, Bush, Siegal, Provencio, “Synthesis of    Large Arrays of Well-Aligned Carbon Nanotubes on Glass,” Science,    282:1105, 1998.-   2. Merkulov, Lowndes, Wei, Eres, Voelkl, “Patterned Growth of    Individual and Multiple Vertically Aligned Carbon Nanofibers,” AppL    Phys. Lett., 76:3555, 2000.-   3. Ren, Huang, Wang, Wen, Xu, Wang, Calvet, Chen, Klemic, Reed,    “Growth of a Single Freestanding Multiwall Carbon Nanotube on Each    Nanonickel Dot,” Appl Phys. Lett., 75:1086, 1999.-   4. Baker, “Catalytic Growth of Carbon Filaments,” Carbon, 27:325,    1989.

1. An apparatus, comprising an expanded base carbon containing tipincluding: a carbon containing expanded base coupled to a substrate; anda carbon containing extension coupled to said carbon containing expandedbase, wherein said carbon containing expanded base is substantiallycylindrically symmetrical and said carbon containing extension issubstantially cylindrically symmetrical, and wherein the expanded basecarbon containing tip defines a slope with a discontinuity between thecarbon containing expanded base and the carbon containing extension. 2.The apparatus of claim 1, wherein said carbon containing expanded baseis substantially conical.
 3. The apparatus of claim 2, wherein saidcarbon containing expanded base defines a substantially solid cone. 4.The apparatus of claim 2, wherein said carbon containing expanded basedefines a substantially hollow funnel.
 5. The apparatus of claim 1,wherein said carbon containing extension is substantially cylindrical.6. The apparatus of claim 5, wherein said carbon containing extensiondefines a substantially solid rod.
 7. The apparatus of claim 5, whereinsaid carbon containing extension defines a substantially hollow tube. 8.The apparatus of claim 1, further comprising another expanded basecarbon containing tip coupled to the substrate, the another expandedbase carbon containing tip including another carbon containing expandedbase coupled to said substrate; and another carbon containing extensioncoupled to the another carbon containing expanded base.
 9. An electronemitter, comprising the apparatus of claim
 1. 10. The apparatus of claim1, wherein the carbon containing expanded base includes a graphiticcarbon film.
 11. The apparatus of claim 10, wherein the carboncontaining expanded base includes a precipitated carbon film.
 12. Anapparatus, comprising: a substrate; and a sharp tip carbon nanostructurecoupled to the substrate, wherein the sharp tip carbon nanostructuredefines a tip diameter that is a function of a size of a catalystdroplet, wherein the sharp tip carbon nanostructure includes a carboncontaining expanded base coupled to the substrate and a carboncontaining extension coupled to the carbon containing expanded base, andwherein the carbon containing expanded base is substantiallycylindrically symmetrical and said carbon containing extension issubstantially cylindrically symmetrical.
 13. The apparatus of claim 12,wherein the carbon nanostructure defines a height that is grown to amicron size.
 14. The apparatus of claim 12, wherein the carbonnanostructure defines a base diameter that is grown to a micron size.15. An electron emitter, comprising the apparatus of claim
 12. 16. Anapparatus, comprising: a substrate; and a carbon nanostructure coupledto the substrate, wherein the carbon nanostructure defines a carboncontaining expanded base, wherein there is a connection between thecarbon containing expanded base and the substrate, wherein the carbonnanostructure includes a carbon containing extension coupled to thecarbon containing expanded base, wherein said carbon containing expandedbase is substantially cylindrically symmetrical and said carboncontaining extension is substantially cylindrically symmetrical, andwherein the expanded base carbon containing tip defines a slope with adiscontinuity between the carbon containing expanded base and the carboncontaining extension.
 17. The apparatus of claim 16, wherein the carbonnanostructure includes a carbon nanocone that is characterized bymechanical stability.
 18. An electron emitter, comprising the apparatusof claim 16.